As semiconductor technology progresses, devices formed on semiconductor substrates grow smaller. As devices grow smaller, manufacturers are continually challenged to develop productive processes for making the devices. Currently, manufacturing processes are being deployed to make devices having critical dimension of 45 nm. Researchers are busy developing next generation processes for devices having critical dimension of 20 nm or less. At these extreme dimensions, implanting dopants in a substrate becomes forbidding. In a traditional boron doping process, for example, boron atoms are directed toward a substrate with sufficient energy to penetrate the crystal lattice to a desired depth, and the substrate is then annealed to distribute the boron and activate it (attach it to the crystal network). As device dimensions grow smaller, control of implantation depth becomes more critical. Next generation devices are expected to have junctions no more than about 50 atomic layers deep.
Implantation problems arise as junction depth diminishes. Because the ions must travel more slowly to avoid implanting too deeply, the repulsive charge among like-charged ions forces them to diverge from their intended path. To compensate for this effect, fast-moving ions are magnetically decelerated near the surface of the substrate. Beam deceleration, however, results in “energy contamination,” arising from exchange of charge between fast-moving ions and fugitive neutral particles during or prior to deceleration. The fast-moving neutralized particles are unaffected by the beam decelerator and implant deeply into the substrate.
Small ions also channel through the crystal lattice. Because the crystal lattice has open spaces large enough for many ions to pass unimpeded, more ions will travel down these “channels”, resulting in highly variable implant depth. To reduce the tendency to channel, many manufacturers have resorted to “pre-amorphizing” the substrate surface to remove any opportunity for channeling. Pre-amorphization may also improve implant dose by opening more space within the solid matrix for ions to penetrate. Pre-amorphized substrates require more annealing, however, to activate dopants because the crystal structure is completely disrupted to a considerable depth and must be repaired. This leads to unwanted dopant diffusion and residual EOR damage.
Thus, there is a continuing need for better methods of implanting dopants in a shallow junction with high dopant dose and activation, low sheet resistance, and even distribution of dopants.